A couple of the reviews seem to think that some more earth scenes would have been good, and could have strengthened the overall story arc. They all seem to like Portman, Dennings and Skarsgaard (when he is mentioned) as well.
Overall, to the extent that I'm seeing a common thread in these reviews, on the negative side, it's not so much that the earth scenes are weaker than the Asgard scenes, it's that the transition between the two is a little jarring.
Other than that, everything seems very positive overall, with each reviewer having certain characters and certains aspects that stand out as particularly good. Which ones actually seems to vary quite a bit, though, which I think is an excellent sign.